A ball screw device comprises a screw shaft having a thread groove in the outer peripheral surface thereof and a nut having a thread groove in the inner peripheral surface thereof and externally mounted on the screw shaft. A plurality of balls are interposed between the respective thread grooves of the screw shaft and the nut. In such a ball screw device, the balls circulate between the respective thread grooves according to the rotational movement of the nut or the screw shaft, which causes a relative telescopic motion between the nut and the screw shaft. In this case, it is necessary, irrespective of the telescopic motion, to provide a structure to prevent the balls from slipping out of between the respective thread grooves. One of such structures is a ball circulation mechanism. The ball circulation mechanism couples both ends of a ball passageway shaped by the respective thread grooves of a screw shaft and a nut to form a closed loop, and thereby circulate the balls in the closed loop. The conventional ball circulation mechanism employs a so-called deflector. The deflector is described below.
The deflector, in general, serves to couple the upstream side of the thread groove, which balls are rolled from, and the downstream side thereof, which the balls are rolled to, so that the balls on the downstream side of the thread grooves are returned to the upstream side thereof over a ridge. The deflector is, being fitted into a radial through hole formed in the nut, fixed to the through hole by an adhesive. The deflector has, in the inner diameter surface thereof, a ball circulation groove of meandering shape for returning the balls from the downstream side, over the ridge, to the upstream side in substantially one turn of the thread grooves.
Such a ball screw device requires an externally mounted part, that is the deflector, which generates additional costs for the deflector, processing the through hole for mounting the deflector in the nut and mounting the deflector to the nut. Besides, when the deflector is mounted to the through hole of the nut rather imprecisely, the ball circulation groove and thread groove cannot be precisely positioned, which results in a degraded circulation performance of the balls. Thus, mounting the deflector requires precision, which is costly.
Therefore, a main object of the present invention is to provide a ball screw device capable of circulating balls without using a deflector.
Another main object of the present invention is to provide a ball screw device capable of circulating balls without using a deflector, and therefore without additional costs.